Geology Reference
In-Depth Information
Figure 10.19. Example of synthetic isotope plots from a numerical model of man-
tle dynamics with tracers. The lines and boxes summarise the observations. The
spread of isotope values is sensitive to poorly constrained sampling scales and
melting processes, so these results should only be taken as indicative. From Bran-
denburg
et al.
[219]. Copyright Elsevier Science. Reprinted with permission.
Finally, the synthetic isotope plots constructed from the final model results are
very sensitive to several poorly constrained factors, including the sampling volumes
in the mantle and the degree to which melts mix, which will have a large effect on
the isotopic spread. To those we can add the effects of heterogeneity and associated
disequilibrium, whereas the estimates assume homogeneity and equilibrium, and
even then a number of the required geochemical parameters are not well determined.
Thus, although it is possible to obtain plausible synthetic isotope plots from the
models, they should not be regarded as strong tests of the mantle models. They are,
to a greater degree, tests of our understanding of melting and melt migration. A
more recent example of synthetic isotope plots is shown in Figure 10.19.
C&H found that the apparent age generated in their synthetic lead isotope plot
(1.6-1.8 Ga) was significantly larger than the residence time of tracers in the model
(about 1.4 Ga). The reason is that the apparent age is strongly controlled by the
oldest tracers, which generate the broadest spread of values and thus weigh more
heavily in constraining the linear correlation. Thus models that generate residence
times of 1.4-1.6 Ga can be considered consistent with observations for the purpose
of the following discussion.
Because of these considerations, the most robust observational features are
the differences between MORBs and OIBs and the apparent lead age, corrected
downwards for comparison with residence times in models. The spread in isotopic
ratios is a less reliable measure of mantle sources.
